ON BRITISH FOSSIL REPTILES. 199 



as suddenly as it entered it in the lias, and with every appreciable osteological 

 character unchanged. 



Of the different species of the Ichthyosaurus, founded upon minor modi- 

 fications of the skeleton, several appear contemporaneously in the strata where 

 the genus is first introduced ; and those which remain the longest manifest 

 as little change of specific as of generic characters. There is no evidence 

 whatever that one species has succeeded or been the result of the transmuta- 

 tion of a former species. The tenuirostral Ichthyosaurus existed at the same 

 time, and under the same external influences, as the stronger and shorter jawed 

 Ichthyosaurus communis ; just as the tenuirostral Delphinus Gangeticus co- 

 exists at present with the short-jawed porpoise. 



If the relative periods of existence of the different Enaliosaurian reptiles 

 were not well ascertained, and room were allowed for conjecture as to their 

 successive appearance on this planet, it Avould be as easy as seductive to spe- 

 culate on the metamorphoses by which their organic framework, influenced 

 by varying conditions, during a lapse of ages, might have been gradually 

 modified, so as to have successively developed itself from an Ichthyosaur to 

 a Plesiosaur, and thence to a Crocodfle. 



We may readily conceive, for example, the fish-like characters of the ver- 

 tebral column of the Ichthyosaurus to have been obliterated by a filling-up 

 of the intervertebral cavities through ossification of the intermediate elastic 

 tissue, and the Plesiosaurian type of vertebra to be thus acquired. The normal 

 digits of the fin might be supposed to become strengthened and elongated by 

 more frequent reptation on dry land, and thus to cause an atrophy of the 

 supernumerary fingers : phalanges of a more saurian figure might have been 

 produced by the confluence of a certain number of digital ossicles: the head 

 might be shortened by a stunted growth of the intermaxillary bones, and thus 

 be reduced to Plesiosaurian proportions. The teeth might become more 

 firmly fixed by the shooting of bony walls across their interspaces, as in the 

 young Crocodiles. If we now elongate the bodies of the vertebra, reduce 

 some twenty pairs of anterior ribs to hatchet-bones, place the fore-paddles at 

 a corresponding distance from the head, and the hind-paddles proportionally 

 nearer the end of the tail, little more will be required to complete the trans- 

 mutation of the Ichthyosaur into the Plesiosaur. 



If next, in adaptation to a gradual change of surrounding circumstances, 

 the jaws of the Plesiosaur became lengthened to the proportions of those of 

 the tenuirostral Ichthyosaur, but at the expense of the maxillary, instead of 

 the intermaxillary bones, preserving the socketed implantation of the teeth ; if, 

 to balance the elongation of the jaws the neck at the same time shrank to 

 nearly its former Ichthyosaurian proportions, with some slight modifications 

 of the Plesiosaurian type of the vertebras ; if a further development and a 

 more complete separation of the digits of the fore and hind members were to 

 take place, so that they might serve for creeping as well as swimming ; if the 

 exposure of the surface to two different media, and of the entire animal to perils 

 of land as well as of sea, were to be followed by the ossification of certain 

 parts of the skin, and the acquisition, by this change, of a dermal armour, — 

 such we might conceive to be the leading steps in the transmutation of the 

 Plesiosaur into the Teleosaur. 



And if the three forms of extinct Saurians, whose changes of specific and 

 generic characters have thus been speculated on, had actually succeeded each 

 otlier in strata successively superimposed in the order in which they have 

 here been liypotlietically derived from one another, some colour of probability 

 might attach itself to this hypothesis, and there would be ground for search- 

 ing more closely into the anatomical and physiological possibilities of such 



